Field of the Invention
The present invention concerns a method and a magnetic resonance (MR) system to acquire MR data in a predetermined volume segment of an examination subject with the magnetic resonance system.
Description of the Prior Art
In both MR imaging and MR spectroscopy, it is often necessary to suppress unwanted signal contributions originating from a defined region, defined regions of the examination subject by suitable measures, or to specifically acquire only signal contributions originating from defined regions of the examination subject. For example, a movement (due to a respiration of the examination subject, for example), a flow or a pulsation (in blood vessels, for example) can lead to artifacts that are not only locally limited to the corresponding region but also are visible in the entire MR image, and thus can lead to a reduced diagnostic quality.
A first possibility to restrict these artifacts, and therefore to improve the quality of the MR images, is to excite the volume segment to be examined but not the disruptively affected region with suitable excitation pulses, such that the signal of the disruptively affected region does not enter into the imaging process. The complete volume segment to be examined should be excited (and thus acquired) by the imaging process.
A second possibility to restrict such artifacts (and therefore to improve the quality of the MR images) is to saturate the magnetization of the nuclear spins in the disruptively affected region by suitable preparation pulses, and to thereby reduce the signal contribution from this region in the subsequent imaging process. The signal from the examination region or from the volume segment to be examined should not be negatively affected by the preparation pulses.
In both possibilities, it must be considered that multiple spin species can be present that exhibit a different chemical shift (for example fat and water protons). In the excitation case (the first possibility), the risk exists that chemically shifted spin species may not be excited in the examination volume. In the saturation case (the second possibility), the risk exists that chemically shifted spin species in the examination volume are suppressed. Both situations lead to a reduced diagnostic quality of the MR images since relevant information is not acquired from the volume segment to be examined, and therefore is missing in the created MR images.
A comparable situation can be present when the basic field of the magnet of the magnetic resonance system is statically or dynamically distorted.